1. Field of Invention
The field of art to which this invention relates includes, generally, that of simulator control loading systems and, more particularly, to an improved high performance system for developing realistic reaction forces in manually-operable controls in a vehicle simulator.
This invention is adapted particularly to hydraulic control loading systems for use with a vehicle simulator such as shown in the U.S. Pat. No. 4,236,325, issued Dec. 2, 1980, to John D. Hall et al. and assigned to the same assignee as the present invention.
2. Description of the Prior Art
Today, one of the best and most widely accepted ways of teaching pilots to fly is through the use of an aircraft simulator which permits the accumulation of flight experience without the high risk and the excessive cost that otherwise may be involved. To be a truly effective teaching instrument, however, the flight simulator must provide sensory cues that are realistic to the pilot trainee.
One important contributor to the realism of a simulator is the "feel" associated with the primary flight controls. Since aircraft control surfaces do not exist in a simulator, the forces associated with the surfaces must be produced by artificial means in order to create the necessary "feel" at the control.
Therefore, a system to "load" the control is required if the aircraft's control forces are to be duplicated. Many training judgments depend upon a pilot's response to these forces and to the "feel" of the controls.
For example, during flight operation of an actual aircraft, the pilot must control his ailerons, elevator and rudder to maneuver an aircraft. This control is maintained by a pilot's use of both hands and feet by holding on to a conventional control stick to control the elevator and the aileron systems while stepping on foot pedals to operate the rudder system.
To simulate these controls effectively, forces must be developed in the control stick and in the foot pedals to replace these missing forces. A pilot must "feel" not only these forces but also the effects of his actions as communicated to him through these forces applied to these controls.
As a pilot moves his control stick and adjusts the position of his foot pedals to maneuver his aircraft, he must feel resistance in his hand and his feet, simulating the actual resistance that a pilot flying an actual aircraft would feel. Here the requirement is not only for high performance on the part of the simulator control loading system but also for the capability of verification that the system is within certain specified standards of operation.
In addition, the quest for higher performance in this simulator control loading system has been accompanied by a trend in the industry to move away from subjective acceptability standards to those which have taken over in other areas in simulation; namely, the use of measurement and calibration systems which will ensure a close matching of the simulator control system force and position curves to those which are obtained in actual performance of operational aircraft.
Several different types of control loading systems have been developed in the past and some of these are currently in use in simulators today. Many of these systems employ force feedback techniques in order to produce electrical signals that are representative of the required control forces.
Typically, in a force feedback system, a computer is used to generate an electrical signal corresponding to the control force that is required for any given flight condition. This signal reflects the aerodynamic characteristics of the aircraft being simulated and the particular flight maneuver being performed.
In addition, an electrical signal representing the force exerted on the control by the pilot is generated. The pilot's force signal is fed back and compared with the required force signal. A signal corresponding to the difference between the required force and the pilot's force is generated to obtain proper loading of the control.
U.S. Pat. No. 4,236,325 teaches the use of a servo valve and a hydraulic actuator in combination to provide loading of simulator controls. A signal representing the difference between actual and required forces is used to drive a servo valve which, in turn, provides hydraulic oil flow to an actuator. The magnitude and the polarity of the driving signal determines the amount of hydraulic oil flow to each side of the actuator's piston.
A difference in the amount of hydraulic oil on opposing sides of the actuator's piston will cause the piston to move and exert a force on the piston rod. Therefore, by connecting the controls to the piston rod by appropriate mechanical linkage, the necessary force is transmitted to the controls.
During normal operation of some control loading systems suggested in the past, various forces are produced in order to obtain the proper "feel" in the controls. While some of these forces may be large enough to pose a threat to the safety of a pilot, they must be produced in this magnitude in order to simulate accurately the control's characteristics under certain flight conditions.
On the other hand, force levels above those that can be encountered in an actual aircraft must not be permitted to be transmitted to the controls in the simulator. Therefore, most of the control loading systems in the past do not develop these forces in a magnitude sufficient for obtaining the proper "feel" in these manually-operable controls.
There can be failures in either the hydraulic or the electrical means which make up the control loops to provide the "feel" to simulated aircraft controls. Where the failure is in the hydraulic system and where the electrical controls are still operative, additional electrically controlled elements can usually activate safety devices to protect equipment and personnel.
However, circumstances may conspire to prevent the electrical controls from functioning correctly, or a hydraulic failure may occur too rapidly for effective control by electrical means. For these reasons, most of the control loading systems in the past do not develop performance to the high level that is obtainable consistently and reliably by the present invention.
Among the prior art devices for dealing with this problem by providing hydraulic safety means are U.S. Pat. No. 3,033,174 and U.S. Pat. No. 3,067,725 both of which are in the name of Harold S. Hemstreet and both assigned to the same assignee as the present invention.
In the past, it has been suggested to use electrical controls attached to a servo valve with a relay to actuate the valve. The connection is such that the relay operates the valve in response to changes in electric current flow corresponding to certain changes in pressure which are indicative of hydraulic failure.
Such devices have proven unsatisfactory, however, since they respond only to very large changes in hydraulic pressure and introduce an undesireable time delay. If some of these prior art devices are operated at the high performance level that is needed in the simulator field, the system could be exposed to at least one violent shock before the safety device becomes operative.
Another prior art device proposes simple mechanical stops which are intended to absorb the force that is generated in excess to a desired or safe force. These stops are inadequate for many installations where space requirements are such that the mechanical stops can not be placed in a way to stop the undesired motion adequately. An example of such limited space is to be found in the cockpit space of aircraft simulators.
A pilot trainee is quite sensitive to shock and vibration in the control forces so that it is highly desireable and generally necessary to provide a better high frequency response in a flight simulator control loading system. Since striking a limit stop, for example, may create an audible knock, there are situations wherein such audible sounds detract substanstially from the realism sought to be obtained.
In modern grounded aircraft training systems, it is acknowledged that failthfulness of simulation is required for adequate training, particularly for training persons to operate present day aircraft of increasingly higher performance. It is acknowledged also that pilot familiarity with particular aircraft is based to a considerable degree upon recognition of both the static and the dynamic control forces which the pilot must apply to the aircraft controls in order to perform various maneuvers.
Shortcomings in the control loading systems included with a flight simulator may have an adverse effect upon the simulator flying qualities, with a result of material decrease in the validity of training. A control loading system must provide a force-generating system of considerable force capability which must have a realistically smooth "feel" and have extremely small friction, except for the small amount of friction present in an actual aircraft system. The generation of large hydraulic forces also have attendant risk of control unit damage through overloading or overheating.
Very small amounts of aileron or elevator friction can cause serious difficulty in maintaining an aircraft trimmed in flight, particularly at high speeds. When a control loading system is used as a part of an autopilot or other closed-loop control system, it is mandatory that the system meet certain dynamic requirements connected with the over all stability of the complete autopilot system.
Today's high standards of simulation require that the control loading unit be capable of generating non-linear force verses displacement curves, break-away forces or detent effects (corresponding to similar effects deliberately introduced into aircraft control systems), and limiting of stick travel as the result of limiting aerodynamic hinge movements or reduced hydraulic system capabilities during emergency operation.
In flight simulator control loading systems, it is necessary to generate forces on the manually-operable controls that are proportional, other factors, to such variables as control displacement, control trim positon, aerodynamic parameters, autopilot forces, etc. In artificial force-producing systems, a large variety of factors determine the magnitude of force in existence at a particular instance on the control.
Utilizing the present invention, the various nonlinearities and the rigid dynamic requirements for an aircraft flight simulator can be met without difficulty through electronic circuitry, as will be described in more detail presently. Thus, there has existed a long felt need for a means to detect conditions that are capable of producing unrealistic or incorrect forces at the manually-operable controls and to abort the control loading system before any incorrect force can be introduced to give the operator-pilot wrong information.